state_names (list of str) – States are similar to data and label, but not provided by data iterator.
Instead they are initialized to 0 and can be set by set_states()

group2ctxs (dict of str to context or list of context,) – or list of dict of str to context
Default is None. Mapping the ctx_group attribute to the context assignment.

compression_params (dict) – Specifies type of gradient compression and additional arguments depending
on the type of compression being used. For example, 2bit compression requires a threshold.
Arguments would then be {‘type’:‘2bit’, ‘threshold’:0.5}
See mxnet.KVStore.set_gradient_compression method for more details on gradient compression.